1. Field of the Invention
The invention relates to a hydrodynamic working circuit having at least two bladed wheels, and more particularly to a retarder having means effective in shut-off condition for avoiding losses due to an undesirable air flow or circulation in the working chamber of the working circuit, which in such shut-off condition is filled with air.
2. Related Art
In hydrodynamic brakes (retarders), capacity losses occur when the retarder is in emptied condition due to air circulation between the two blade rims, when there is a difference in rotational speed therebetween. From U.S. Pat. No. 3,072,222 (equivalent to DE-PS 11 44 317) it is known that a partition wall (similar to a camera diaphragm) may be inserted into the air gap between stator and rotor for providing a substantial reduction of these losses, by preventing such air circulation.
From U.K. Patent Specification 1,365,670 (equivalent to DE-PS 22 27 624) is known another arrangement in which resiliently slidable tappets are arranged on the blades of one bladed wheel, which, during operation of the fluid circuit, are pressed by the working medium out of the flow profile, and then dip into the working chamber after emptying, and thus form a hindrance to the flow of circulating air.
Furthermore, from U.K. Patent Specification 1,342,228 (equivalent to DE-PS 21 35 268) is known an arrangement wherein hinged flow hindrances are pressed out of the working chamber during operation of the fluid circuit, and after emptying close automatically in the external area of the working chamber. The evacuation of the working chamber, too, has been suggested (DE-PS 677 160, DD-PS 63 941).
A further system of interest is disclosed in DE-PS 97 934.
Thus, there are well-known means by which the power-consuming air ventilation in switched-off condition can be reduced. These solutions, however, require, for example in the case of DE-PS 11 44 317, a complicated automatic system in which, on emptying of the working chamber, the diaphragms are slid in, necessitating an expensive linkage and guiding of the diaphragm components as well as a separation of the blade rims so that the diaphragm components can be slid in contact-free. This reduces the braking capacity. In the case of the other documents mentioned above, flow hindrances are pressed out of the working chamber during operation only when the flow is sufficiently intensive, and thus capacity losses are to be expected at low speed.
From DE-OS 32 17 465, a system is known with which unacceptable heating of a hydrodynamic coupling in emptied condition is prevented. For this purpose a certain quantity of fluid is continuously introduced into the working chamber as coolant. In interaction with the enclosed air, a partial vacuum is generated. The coolant is intended to absorb the heat contained in the enclosed air and to exit again through valves at the periphery of the coupling. The aim is apparently to dissolve the air in the working chamber of the coupling in the coolant. For this purpose, however, the coolant must, when it has left the working chamber of the coupling, be returned to an open vessel at atmospheric pressure so that the air can escape from the coolant. The coolant must consequently be fed into the working chamber air-free. It is doubtful whether the coolant, under the conditions of a partial vacuum, is capable of absorbing air at all. It is, furthermore, necessary to make openings at the rotating bladed wheel ahead of the trailing edge of the blades so that the blade channels formed by the blades are open radially outward at these points.
This design of blades for a retarder is disadvantageous because it decisively reduces the braking torque that has developed. Furthermore, the supply of the working medium to a retarder is generally effected in a closed circuit which makes it impossible for air to escape from the working medium. Rather, a retarder is generally operated with a fluid-air mixture, with the working chamber of the retarder being subjected to overpressure, so that the air remains dissolved in the working medium and has no negative influence on the mode of operation of the retarder. A system as per DE-OS 32 17 465 is therefore not usable for a retarder.